For convenience of description the present invention will be described with reference to the UMTS system.
A typical UMTS system architecture is shown in FIG. 1 of the accompanying drawings and comprises at least one primary (or base) station PS and a plurality of secondary stations, generally referred to as User Equipments (UEs), UE1, UE2. Communication between the primary station PS and each of the Ues is by radio links consisting of a downlink from the primary station PS to a UE and an uplink from the UE to the primary station PS.
A data packet protocol is used for controlling the transmission of data packets between the or each primary station PS and the UEs in the relevant service area. In UMTS the uplink data packet standard is still evolving and with respect to High-Speed Uplink Packet Access (HSUPA) the latest version of the MAC specifications is 3GPP document 25.321. FIG. 2 of the accompanying drawings illustrates that with HSUPA each UE can have up to eight active Hybrid Automatic Repeat Request (HARQ) processes HARQ 1 to HARQ 8, which are transmitted in turn in successive Transmission Time Intervals (TTIs) TTI 1 to TTI 8 (i.e. synchronous HARQ) on an uplink (UL). For convenience of description the group of TTIs 1 to 8 will be collectively referred to as a frame FR. Depending on whether or not a data packet is received satisfactorily, a scheduler in the primary station PS causes a positive or negative acknowledgement ARQ1 to ARQ 8 to be transmitted on the downlink (DL) and in response the UE either sends a new data packet or resends the previous data packet. An uplink transmission can also comprise an uplink control signal.
The transmission rate for these processes is set according to a “Serving Grant” (SG) variable which is stored in the UE. The SG is a record of at what rate and/or power and/or power ratio a UE can transmit at until a new grant is received from the primary station. The value of the SG can be updated by “all-process” (i.e. common) absolute grants (AG), or by process-specific “single-process” absolute grants, or by relative grants (RG). An AG gives an indication of the new SG whereas a RG is an indication of an incremental or decremental change to SG relative to the data transmission rate in the correspondingly numbered TTI in the previous frame FR.
All-process absolute grants (AG) change the value of the SG without affecting which of the processes are active, whereas single-process absolute grants set the indicated process to active or inactive as well as updating the value of the SG. The nature of absolute grants (all-process or single-process) is indicated by an “all process” flag which is sent together with the grant value.
Relative grants (RG) are associated with a particular HARQ process by means of a predetermined timing relationship. A relative grant sets the SG relative to the data transmission power and/or rate used for the previous transmission of the HARQ process in question. Note that, as this sets the SG, the implementation of a RG also affects subsequent transmissions on different HARQ processes in the same way as AGs.
UEs maintain a Serving Grant and the list of active HARQ processes based on the received absolute grant and relative grant commands. Each absolute grant or relative grant command is applied at a specific TTI.
The above behaviour can give undesirable results in some cases when absolute and relative grants are received in sequence.
FIG. 3 of the accompanying drawings shows a case when a UE's SG is set at a first value referenced 30 in TTI1, during which TTI the UE transmits data using HARQ process 1, using the maximum data transmission power (or rate in some embodiments) indicated by the SG. In subsequent TTIs, the UE may transmit data using the corresponding consecutively-numbered HARQ processes.
For TTI 2, the UE receives an AG to reduce the SG to a second value referenced 32. For TTI 9 (which is the first TTI in the next following frame), the network wishes to reduce the UE's data transmission power by a further 1 dB, but without the relatively high signalling overhead associated with sending a AG. Normally a RG would be the appropriate way to achieve such a reduction in data transmission power with a low signalling overhead, but in this case a “down” relative grant would be applied relative to the transmission power actually used in TTI 1, that is the SG level 30, which would therefore result in an unwanted increase in the data transmission power, to a level 34, compared to the SG level 32 used in the previous 7 TTIs (which followed the constraint imposed in TTI 2 by the absolute grant), that is TTI 2 to TTI 8.
According to the current behaviour, the network therefore has no way to reduce the SG in TTIs 2-9 without using another AG with its associated overhead in signalling (or waiting till TTI 10).
Likewise, the network has no way to increase the SG by one step (e.g. +1 dB in some embodiments) relative to the value in TTIs 2-9, as an “up” RG of 1 dB in TTI 9 could cause the SG to be raised by much more than one step or increment relative to the SG level 32, to a level 36, as shown in FIG. 4 of the accompanying drawings.
One way of describing this behaviour is to say that the selection of the reference value for the RGs gives priority to RGs over AGs. This may not always be appropriate or desirable.